1. Field of the Invention
The present invention is directed to pressure sensors that are sufficiently robust to be used in harsh environments, such as an oil or gas well. More specifically, the invention is directed to a rugged pressure sensor that incorporates a Fabry-Perot optical cavity with an optical fiber to provide a pressure sensor capable of sensing pressure and pressure changes by analyzing the light reflected by the Fabry-Perot cavity.
2. Description of Related Art
There are many processes and environments in which it is desirable to know the ambient pressure. One such common process or environment is during exploration and production of hydrocarbons such as oil in which it is necessary to measure the pressure of the hydrocarbons in a reservoir. Another application is the measurement of the fluid pressure associated with pumps or natural drivers for transporting such hydrocarbons from one location to another. One way that fluid flow detected senses the pressure drop across a venturi, thus requiring detection of the pressure difference on both sides of the venturi.
Pressures of fluids are typically measured with, for example, a quartz crystal based pressure measuring device such as the Quartzdyne™ Series QS High Pressure Laboratory Transducer manufactured by Quartzdyne, Inc. of Salt Lake City, Utah. Such a pressure sensing device measures the change in mechanical oscillation frequency associated with the elastic deformation of the quartz crystal in response to applied pressure. Quartz is the medium of choice for such applications due to inherent long term stability, as well as its minimal creep and hysteresis properties. The change in frequency with temperature is also very predictable.
Traditionally the change in frequency of the quartz crystal is measured and compared to a reference crystal which is temperature compensated with the resulting data correlated and calibrated to a direct pressure measurement. Although the reliability of such a quartz crystal is extremely high, the electronics required to measure frequency change are subject to failure particularly when the transducer and its associated electronics are subjected to elevated temperatures, such as temperatures above 125° C.
Pressure sensors with optical fibers and optical sensing elements are very important for remote sensing where conventional gauges cannot operate or where conventional gauges are not reliable enough.
Certain techniques exist for measuring pressure using a Bragg grating. However, such techniques are either complex, costly, or do not constrain the optical fiber from buckling in the grating region. For example, a fiber optic grating based sensor is described in U.S. patent application Ser. No. 08/925,598 entitled “High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments.” In that example, an optical fiber is attached to a compressible bellows at one location along the fiber and to a rigid structure at a second location along the fiber with a Bragg grating embedded within the fiber between the two fiber attachment locations. The fiber is attached at both locations so as to place the grating in tension. As the bellows is compressed due to an external pressure change, the tension on the fiber grating is reduced, which changes the wavelength of light reflected by the grating. Such a sensor requires a complex bellows structure and does not constrain the fiber from buckling in the grating region.
Another example of a fiber grating pressure sensor is described in Xu, M. G., et al, “Fibre grating pressure sensor with enhanced sensitivity using a glass-bubble housing”, Electronics Letters, 1996, Vol. 32, pp. 128-129. In this example, an optical fiber is secured by UV cured cement to a glass bubble at the fiber's two ends, with a grating formed in the fiber at a location situated inside the bubble. However, such a sensor does not constrain the optical fiber against buckling in the region of the grating.
It is also known that a grating-based pressure sensor may be made by placing a polarization maintaining optical fiber in a capillary tube having rods therein, and measuring changes in grating birefringence caused by changes in the transverse strain on the fiber grating due to transverse pressure forces acting on the capillary tube, as is discussed in U.S. Pat. No. 5,841,131. However, such a technique may be difficult and expensive to implement.
Another limitation of the use of fiber Bragg gratings as integral parts of a pressure sensor is that the tension on the optical fiber of such a sensor cannot be very great without breaking the fiber. Accordingly, such a sensor may have a limited range of sensitivity.
Attempts have been made to use Fabry-Perot optical cavity sensors to measure pressure. Typically, sensors of this type rely upon a polished optical fiber as one reflector of the optical cavity and a flexible diaphragm as the other reflector of the cavity, and a gap between the two. As the pressure increases in such a sensor, the diaphragm deflects, changing the gap. One disadvantage of this construction is that the diaphragm, when it deflects, does not remain perfectly flat as the pressure changes, since at least the periphery of the diaphragm is typically attached to the sensor housing to hold the diaphragm. This provides multiple reflective paths that distort the spectrum of the light within the Fabry-Perot optical cavity, resulting in measurement errors.
Other Fabry-Perot pressure sensors have been described using two optical fibers placed in opposite ends of a glass ferrule, then bonded in place with a desired gap. The glass ferrule must be of sufficient strength and thickness to withstand high external pressures. If the fibers are bonded inside the ferrule along their entire length, only the area of the gap, typically on the order of 100 microns, will be affected by the applied pressure, which provides very low sensitivity. Attempts to improve the sensitivity of such devices by bonding the fiber only partially along its length, leaving a portion of the fiber adjacent to the gap free, results in erroneous measurements due to the unbonded portion of the optical fiber sticking or slipping in the ferrule as the pressure changes. Moreover, the unbonded sections may move excessively in response to vibrations, resulting in increased measurement error.
What has been needed, and heretofore unavailable is a relatively inexpensive, yet rugged and temperature stabile, sensor for measuring pressure in a harsh environment, such as, for example, a well bore, that overcomes the problems of electronic quartz gauges or sensors incorporating fiber Bragg gratings. Such a sensor would isolate the electronics and other equipment necessary to analyze signals representing changes in pressure within the well bore from the harsh environment of the well. The present invention satisfies these, and other needs.